1. Field of the Invention
The present invention relates to a slip ring device for transmitting electric power or electric signals between a stator and a rotor of a rotary electric machine.
2. Description of the Related Art
A typical rotary electric machine is a electric power apparatus such as an electric motor or a generator, or a medical treatment apparatus such as an x-ray CT scanner. This type of rotary electric machine comprises a stator and a rotor. In general, the stator and rotor are provided with a slip ring device. The slip ring device functions to transmit electric signals or electric power between the stator and rotor.
A conventional slip ring device mounted in an x-ray CT scanner will now be described with reference to FIGS. 1 and 2. As is shown in FIG. 1, a slip ring device 10 comprises a cylindrical body 20 mounted on a rotor (not shown), and a brush unit 30 constituted by an n-number of brushes 32 and a block 34 mounted on a stator (not shown). In the following description, the n-number of brushes 32 are represented by three brushes 32a, 32b and 32c.
An x-ray tube 110 is fixed to the rotor (not shown). Accordingly, the x-ray tube 110 and the cylindrical body 20 rotate as one body. The cylindrical body 20, as shown in FIG. 2, includes an insulating resin cylindrical member 22, and an n-number of metal rings 24 arranged in parallel on the outer periphery of the cylindrical member 22 and distanced from one another. In the following description, the n-number of metal rings 24 are represented by three metal rings 24a, 24b and 24c.
The brushes 32a, 32b and 32c are always put in contact with the metal rings 24a, 24b and 24c during rotation of the cylindrical body 20. The brushes 32a, 32b and 32c are attached to the block 34. The block 34 is connected to a low voltage power supply 114 via an n-number of cables 40. In the following description, the n-number of cables 40 are represented by three cables 40a, 40b and 40c.
An n-number of electrically conductive screws 26 are penetrated from the metal rings 24a, 24b and 24c through the insulating resin cylindrical member 22, as shown in FIG. 2. In the following description, the n-number of screws 26 are represented by three electrically conductive screws 26a, 26b and 26c. First end portions of an n-number of cables 42 are connected to the tip end portions of the screws 26a, 26b and 26c. Second end portions of the cables 42 are connected to the x-ray tube 110. In the following description, the n-number of cables 42 are represented by three cables 42a, 42b and 42c.
With the above structure, even when the cylindrical body 20 and the x-ray tube 110 are rotated, electric power can easily be supplied from the low voltage power supply 114 to the x-ray tube 110 through the cables 40a, 40b and 40c, block 34, brushes 32a, 32b and 32c, metal rings 24a, 24b and 24c, electrically conductive screws 26a, 26b and 26c, and cables 42a, 42b and 42c.
In order to ensure good electrical connection between the low voltage power supply 114 and x-ray tube 110, as stated above, it is necessary to enhance electrical characteristics of the cylindrical body 20. In addition since the cylindrical body 20 is continuously rotated, wear resistance of the cylindrical body 20 must be increased.
To solve this problem, a silver plating has conventionally been applied to the metal rings 24a, 24b and 24c, or metal rings 24a, 24b and 24c made of a silver-based alloy have been employed.
The silver plating, however, incurs pollution problems, and, if the diameter of the silver-plated cylindrical body 20 is increased by the plating layer, the manufacturing cost increases.